Abstractions from Precipitation

# Abstractions from Precipitation | Engineering Hydrology - Civil Engineering (CE) PDF Download

### Evaporation and its Measurement

Evaporation is a cooling process in which the latent heat of evaporation of about 585 cal/gm is provided by the water body. in this process liquid changes into gaseous phase at free surface, below the boiling point through the transfer of heat energy.

Dalton’s Law
The rate of evaporation is proportional to the difference between the saturation vapour pressure at the water temperature, es and the actual vapour pressure in the air ea thus
E = K(e- ea)
Where, E = Rate if evaporation (mm/day)
es = Saturation vapour pressure of air (mm)
ea = Actual vapour pressure of air (mm)
e- ea Saturation deficiency

Measurement of Evaporation

1. ISI standard pan
Lake evaporation = Cp × pan evaporation
Where, Cp pan coefficient
= 0.8 for ISI pan
= 0.7 for class A-Pan
2. Empirical Evaporation Equations (Meyer’s Formula) E = km(e- ea)[1 + (V/ 16]
Where, km = Coefficients which accounts for size of water body.
= 0.36 (for large deep water)
∼ 0.50 (for small and shallow waters)
es = Saturation vapour pressure of air in mm of Hg.
ea = Actual vapour pressure of overlying air in mm at Hg at specified height of 8 m.
V9 = monthly mean wind velocity in km/hr at about 9 m above the ground level.

1/7th power Law
V1 / V2 = (H1 / H2)1/7
Where, V1 is the wind velocity at height H1 and V2 is the wind velocity at height H2.

Water Budget Method
This is simplest method bit it is least reliable it is used for rough calculation, it is based on mass conversation principle.
P + Vis + Vig + Vog + E + ΔS + TL
Where, P=Daily precipitation on the water surface.
Vis = Daily surface inflow into take.
Vos = Daily surface outflow from lake.
Vig = Daily underground inflow into the lake.
Vog = Daily underground outflow from the lake.
E = Daily Evaporation
ΔS = change in storage of lake
= +ve if increase in storage
= -ve if decrease in storage
TL = Daily transpiration loss from the plants on the lake.

Energy Budget Method
The energy budget method is an application of the law of conservation of energy. The energy available for evaporation is determined by considering the incoming energy. Outgoing energy and energy stored in the water body over a known time interval.

Where, Hn = Net heat energy received by the water surface
Hn = Hc(1 - r) - Hb
Hc(1 - r) = incoming solar radiation into a surface of reflection coefficient, r
Hb = Back radiation from water body
Hg = Heat flux into the ground
HS = Heat stored in water body
H= Net heat conducted out the system by water flow (advected energy)
β = Bowen’s ratio
δ = Density of water
L = Latent heat of evaporation.

Evapo-Transpiration
While transpiration takes place, the land are in which plants stand also lose moisture by the evaporation of water from soil and water bodies. In hydrology and irrigation practive, it is found that evaporation and transpiration processes can be considered advantageously under one head as evapo-transpiration.
The real evapo-transpiration occurring in a specific situation is called actual evapo-transpiration (AET).

### Penman’s Method

Penman’s equation is based on sound theoretical reasoning and is obtained by a combination of the energy balance and mass transfer approach.

Where, PET = daily evaporation in mm/day.
A = slope of the saturation vapour pressure v/s temperature curve at the mean air temperature in mm of Hg per °C.
Hn = Net radiation in mm of evaporable water per day
Ea = Parameter including wind velocity and saturation deficit.
γ = Psychometric constant
= 0.49 mm of Hg/°C
It is based on mass transfer and energy balance.

Transpiration Loss (T)
T = (w+ w2) - w2
Where, w1 = Initial weight of the instrument
W = Total weight of water added for full growth of plant.
w2 = Final weight of instruction including plant and water
T = Transpiration loss.

Stream Flow Measurement
Streamflow representing the runoff phase of the hydrologic cycle is the most important basic data for hydrologic studies.
Streamflow measurement techniques can be broadly classified into two categories as:

• Direct determination and
• Indirect determination.

Under each category there are a host of methods. The important ones are listed below:

1. Direct determination of stream discharge
(a) Area velocity methods
(b) Dilution techniques
(c) Electromagnetic method and
(d) ultrasonic method
2. Indirect determination of streamflow
(a) Hydraulic structures, such as weirs, flumes and gated structures, and
(b) Slope- area method

### Determination of Velocity

1. Float Method: Float are generally used to determine approximate velocity of the surface, these are floating devices which are passed with the water along the flow of stream.
Vs = L / t Here, Vs = surface velocity
L= Distance travelled by the float in time ‘t’.
2. Current Meters Method: These consists of rotating elements which rotate due to reactions of stream currents. The number of revolution per second are counted. This can be used to measure point velocity of ant depth.
V = aNs + b
Where, V = point velocity
Ns = Number of revolution per sec. a and b are current meters constant.

Velocity Distribution

1.
For turbulent flow
Where, mean velocity
Vs = surface velocity
2. For shallow streams
where  point velocity at 0.6 y from surface
3. For Deep streams
Sounding Weight
W = weight in newton
Average stream velocity
Y = Depth of flow in meters.

Stream Flow (discharge measurement)

1. Area Velocity Method: This method of discharge measurement consists essentially of measuring the area of cross-section of the river at a selected section called the gauging site and measuring the velocity of flow through the cross-sectional area. The gauging site must be selected with care to assure that the stage-discharge curve is reasonably constant over a long period of about a few years.
Total discharge,

2. Moving Boat Method: in this method a special propeller type current meter which is free to move about a vertical axis is towed in a boat at a velocity VB at right angles to stream flow. If the flow velocity is VF the meter will align itself in the direction of the resultant velocity VR making an angle θ with the direction of the boat. Further. The meter will ragister the velocity VR if VB IS normal VF.
VB = VRcosθ
VF=VRsinθ

Where, qi = Discharge through the ith segment.
qi = ((Yi-1 + yi) / 2)V2R sinθ.cosθ.ti
ti = Time required to pass the boat through ith segment.
3. Dilution Method: the dilution method of flow measurement, also known as the chemical method depends upon the continuity principle applied to a tracer which is allowed to mix completely with the flow.

This technique in which Q0 is estimated by knowing C1, C2, C0 and Q1 is known as constant rate injection method or plateau gauging.
Where,
Q0 = Discharge of stream
C0 = Tracer intensity initially
C1 = Tracer intensity at (1)
C2 = Tracer intensity at (2).
Q1 and Q2 are discharge at (1) and (2) respectively.
4. Slope Area Method: it is a very versatile indirect method of discharge estimation and requires
(i) the selection of a reach in which cross-sectional properties including bed elevations are known at its ends.
(ii) the value of Manning’s n and
(iii) water-surface elevations at the two end sections.
(a)Where Hf = Frictional losses
He = Eddy losses
(b)
ke = Eddy loss coefficients.
(c)
(d)
(e)  Where, Q = stream flow (m3/sec)
(f) k = (k. k2... kn)1/n Where, k = Conveyance
(g) k = 1/n . A . R2/3 Where, A = area (m2)
R = Hydraulic mean depth
= Area / wetted perimeter
The document Abstractions from Precipitation | Engineering Hydrology - Civil Engineering (CE) is a part of the Civil Engineering (CE) Course Engineering Hydrology.
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## FAQs on Abstractions from Precipitation - Engineering Hydrology - Civil Engineering (CE)

 1. What is precipitation in civil engineering?
Precipitation in civil engineering refers to the process of water falling from the atmosphere to the Earth's surface in the form of rain, snow, sleet, or hail. It plays a crucial role in the design and analysis of infrastructure projects as it directly affects drainage systems, flood control, erosion, and overall durability of structures.
 2. How does precipitation impact civil engineering projects?
Precipitation has significant implications for civil engineering projects. Excessive rainfall can lead to increased water levels in rivers, causing flooding and damaging infrastructure. It can also lead to soil erosion and instability, affecting the stability of foundations and slopes. Precipitation patterns are crucial in determining the design of drainage systems to effectively manage stormwater runoff.
 3. What are the methods used in civil engineering to measure precipitation?
Civil engineers use various methods to measure precipitation. The most common method is using rain gauges, which collect and measure the amount of rainfall at a specific location. Other techniques include weather radars that track precipitation patterns over a larger area and satellite-based remote sensing to estimate rainfall intensity and distribution.
 4. How does climate change affect precipitation in civil engineering?
Climate change has the potential to alter precipitation patterns, intensify rainfall events, and increase the frequency of extreme weather events like storms and hurricanes. This directly impacts civil engineering projects by necessitating the reassessment of design standards, considering changing rainfall patterns, and incorporating climate change projections to ensure infrastructure resilience and adaptation.
 5. What are the design considerations for civil engineering projects related to precipitation?
When designing civil engineering projects, precipitation-related factors need to be considered. This includes estimating the design rainfall intensity, analyzing and designing drainage systems to handle stormwater runoff, accounting for potential flooding risks, incorporating erosion control measures, and considering the impact of precipitation on soil properties and stability for foundations and slopes.

## Engineering Hydrology

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